1. Field of the Invention
The present invention relates in general to a phase shift mask(PSM). In particular, the present invention relates to a new alternative phase shift mask for canceling phase conflict.
With the trend of highly integrated semiconductor chips, the resolution provided by a photolithography technique has to be improved, and therefore the mask technique of patterning semiconductor chips has also developed.
2. Description of the Related Art
For example, a phase shift mask, in contrast to a conventional mask, comprises a phase shift layer. Therefore, when an exposure light source passes through phase shift layer regions and non phase shift layer regions to reach the positive photo resistance of a semiconductor substrate, a phase difference of 180 degree is generated. This interference generates an offset to the boundary of to-be-shifted patterns so as to further improve the contrast of the pattern.
There is an alternative phase shift mask as shown in FIG. 1, wherein a phase shift layer region 14 and a non phase shift layer region 16 of different depths are formed by alternatively etching a portion of a transparent substrate 10 on the mask which is traditionally constructed of a chrome layer 12 and a transparent substrate 10. The phase shift layer region 14 is in 180 degrees phase represented by the symbol xcfx80, and non phase shift layer region 16 is in 0 degrees phase. Therefore, when the exposure light source 18 passes through the phase shift layer region 14 and non phase shift layer region 16, a phase difference of 180 degrees is generated.
Further, there is a half-tone phase shift mask (half-tone PSM) or an attenuated phase shift mask (attenuated PSM, APSM), as shown in FIGS. 2a-2c. The surface of the APSM is mainly divided into two regions, which are the wholly transparent region 24 in 0 degree phase and the attenuated transparent region 22 in 180 degree phase. The wholly transparent region 24 is mainly constructed of quartz, and the attenuated transparent region 22 has an extra MoSiON layer. The transparency of the wholly transparent region 24 is close to 100%. The transparency of the attenuated transparent region 22, much less than that of the wholly transparent region 24, is within 10%, possibly less than 4 or 6%. As shown in FIG. 2b, it can be seen from the electric field E of the exposure light sources 28 distributed on the positive photo resist of the semiconductor substrate that the exposure light source 28 has 180 degree phase difference between passing through the attenuated transparent region 22 and passing through the wholly transparent region 24. As shown in FIG. 2c, since the exposure light source exposed on the positive photo resist of the semiconductor substrate has a light intensity I proportional to the square of the electric field E. Therefore, the contrast between the wholly transparent region 24 and the attenuated transparent region 22 can be clearer, so that the resolution of the exposure process can be improved.
Refer to FIG. 3. There is a problem incurred when the above mask is manufactured. Since there are lights of two opposite phases during exposure, there is an area having a light intensity of zero corresponding to the border between the phase shift layer region and the non phase shift layer region or the wholly transparent region and the attenuated transparent region on the semiconductor chip. Therefore, when a mask of this kind is applied to the positive photo resist 30, a ghost line 32 is formed.
There are two traditional ways to eliminate the ghost line. The first option is to manufacture a multi-phase stair-step PSM, wherein a variety of regions in different phase differences, such as transparent regions in phases of 0, 60, 120, 180 degrees in sequence, are formed on the mask so as to alleviate a ghost line phenomenon generated during exposure. However, there are problems incurred. That is, not only is mask complexity increased due to laterally increased regions in different phase differences, but also the occupied area, which is accordingly increased, is opposed to the trend of shrinking a semiconductor chip pattern.
The alternative is double exposure. That is, a first mask is used to perform a first exposure so as to form a desired pattern on a positive photo resist. Then, the desired pattern that is already formed is shielded and a second mask is used to perform a second exposure so as to eliminate unwanted patterns such as ghost lines. However, running time is greatly increased. Also, if the pattern tends to shrink, it is a handicap to the alignment between the first and the second masks.
Accordingly, it is a principal object of the present invention to manufacture an alternative PSM for eliminating a phase conflict phenomenon on the border between two different phase regions.
According to the above objectives, a method of manufacturing an alternative PSM of the present invention includes the following steps: first, a translucent layer and a light-shielding layer are sequentially formed on a transparent substrate; then, the light-shielding layer is defined to form a light-shielding pattern; then, the light-shielding pattern is used as a mask to etch the translucent layer to the transparent substrate so as to form a first transparent region in 0 degree phase; a first transparent region is defined to alternatively form a second transparent region in 180 degree phase; the translucent layer is left on the border between the first transparent region in 0 degree phase and the second transparent region in 180 degree phase to form a third translucent region in 85-95, generally 90 degree phase.
A light intensity buffer region formed by the third translucent region in 85-95 degree phase can cancel a phase conflict on the border between the first transparent region in 0 degree phase and the second transparent region in 180 degree phase, and further eliminate ghost lines which are generated during exposing the positive photo resist layer on the semiconductor substrate.